# What Causes Friction

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I was wondering what friction was caused by and why rough textured create more friction then smooth ones.

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Friction is not yet fully understood and quite possibly not due to a single mechanism.

It is possible to develop quite accurate mathematical theories for some surface contacts, but every theory seems to have some exceptions as well as successes.

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I was wondering what friction was caused by and why rough textured create more friction then smooth ones.

Rough texture under microscope will look like mountains and valleys. While smooth will be pretty flat.

How to get smooth surface? by f.e.heating and melting material. Liquid material from mountains will fill valleys and they will reduce them self.

If we will send light beam to rough surface material, photons will bounce from mountains, bounce from valleys, perhaps many times, and photons will be going in the all random directions.

Rough material has many surface normal vectors.

Smooth material has quite uniform normal vector.

Normal vector of surface is used to calculate direction of reflected light.

Once one object mountains are attaching to other object mountains, while object is moving, or is under force, there is needed even more force to unlock them.

There can be also attraction between positive ions in one object and negative ions in other object.

Edited by Sensei
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why would smooth rubber create so much friction compared to other smooth objects

could the atoms be interlocking

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why would smooth rubber create so much friction compared to other smooth objects

It's not just the rubber; the other surface you're using to create friction plays a factor. You can also reduce the frictional coefficient of rubber with another substance, like oil or grease.

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Vulcanized rubber has additional sulfur bonds which render it less sticky.

http://en.wikipedia.org/wiki/Vulcanization

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Tribology (the science of friction) is a huge subject.

Yes, indeed rubber surfaces often have an enhanced friction.

But give us a clue, where are you coming from?

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why would smooth rubber create so much friction compared to other smooth objects

Who said rubber is smooth?

Did you look at it by microscope?

Good smooth surfaces have: specular highlights, and reflections like mirror..

Edited by Sensei
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I am 13 i am still in middle school i just really like physics

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No sweat, Tyler, and good on you for enthusiasm.

OK so I said friction has several causes.

Imagine two surfaces coming close together, but not quite touching.

Say we are placing a block of something on a table.

As we bring them together there must be a first high point that touches.

So all the weight of the block will be supported on this one point of contact.

So the point of contact squashes out since it is not strong enough.

As it squashes the block lowers and a second and then a third etc point of contact occurs.

This keeps happening until the contact area is just enough to support the weight of the block.

This accounts for the first observaion of friction.

Friction is independent of the surface area of the contact face of the block.

(Simply becasue not all the block is in contact).

Depending upon the materials of the block and table the contact points may actually be semi welded together.

Now imagine trying to push the block sideways.

You will encounter a force of resistance.

Part of this resistance is due to the bond between the materials touching.

But perhaps this is partly due to interlock (some high points sticking down into low points and catching when you push sideways.

This second mechanism accounts for why rougher surfaces have greater friction.

Finally the above assumes the block and the table material are rigid.

But if they are flexible then energy is lost flexing the touching points, like a toothbrush.

Is this a good start?

Edited by studiot
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[...] why rough textured create more friction then smooth ones.

This is not the case. There is no simple relationship between rugosity and the coefficient of friction, and some plastics are known to glide more easily against a steel surface that isn't too smooth, and also to wear less then.

Steel and chromium surfaces can be made very smooth: 0.3µm is moderately smooth for steel, and chromium at hydraulic cylinders is hugely better. They shine because they're optically smooth, simply.

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Friction isn't understood - or uncompletely, as a nice wording - despite a good theory would be extremely useful. Friction shouldn't be modelled as climbing hills of rugosity: this old attempt (Newton?) fails to explain low coefficients of friction unrelated with steep slopes. To the very least, such attempts must consider that elastic materials push forward at the places they descend slopes, thus recovering much of the force used to climb the hills.

Some theories have partial success, but they include so many effects with so many tinkering coefficients that they would explain wrong curves as easily as good ones. Anyway, they can only interpolate curves between many measured points, and a graphical interpolation would do it about as well.

A serious difficulty for any theory is that friction depends on surfaces, whose composition can't be inferred from the base material. For instance graphite glides in our atmosphere but rubs in vacuum. Many surfaces meant to glide are so smooth that minimal amounts of allogen materials fill the crevaces.

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• 1 month later...

I think there's no absolute conclusion of friction yet , some scientist say it's the rough surface that causes the friction, while others say it's the forces between molecule . (Actually in tiny scale , noting is perfectly smooth)

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interestingly enough, we now have the question of whether conditions apply.

i suggest a gandy experiment... (thought experiment)

i would first consider eliminating your forces that are possible.

make a list of all the forces involved

make both surfaces as smooth as possible.

start with as simple of an experiment as possible trying to exclude each force involved such as deciding to use material that has no charge build up.

you will be left with only one or two basic things that are directly affecting your measurement.

if you want to see how it works, then you have to take it apart into its most basic pieces.

many think that ideas are created and become stagnated stumps in the process of having that ultimate answer.

experiment wirth it.

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interestingly enough, we now have the question of whether conditions apply.

i suggest a gandy experiment... (thought experiment)

i would first consider eliminating your forces that are possible.

make a list of all the forces involved

make both surfaces as smooth as possible.

start with as simple of an experiment as possible trying to exclude each force involved such as deciding to use material that has no charge build up.

you will be left with only one or two basic things that are directly affecting your measurement.

if you want to see how it works, then you have to take it apart into its most basic pieces.

many think that ideas are created and become stagnated stumps in the process of having that ultimate answer.

experiment wirth it.

I doubt this can be simplified that much. Imagine the dynamics involved in aviation. The range in speeds from takeoff to supersonic and the pressures that are involved everywhere in between these two boundaries. Materials change and respond greatly due to the degree of energy that is being transferred. A jump off a dock as compared to one from a high bridge for example, the same water, two very different results. Materials can stiffen with increasing pressure while others will deform and still others will disintegrate. The considerations of varying temperatures and pressures for each material and their VERY particular and unique dynamic interaction with all other materials . . . . well, just staggers the mind.

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Friction is independent of the surface area of the contact face of the block.

(Simply becasue not all the block is in contact).

Sorry studiot, I dont understand what does "not all the block is in contact" means. Shouldnt the face of the block facing the contact surface will all experience friction once the block is placed on it? And what do you really mean by "all the block"? Can you clarify it?

I am 15 years old and still in secondary school.

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Hello again, Nicholas.

Sorry studiot, I dont understand what does "not all the block is in contact" means. Shouldnt the face of the block facing the contact surface will all experience friction once the block is placed on it? And what do you really mean by "all the block"? Can you clarify it?

I am 15 years old and still in secondary school.

Don't be sorry it is a perfectly good question.

I have drawn some very quick pics to show what I mean.

It is possible to develop this to a numerical theory that predicts friction coeffiients with reasonable accurcy for many metals.

All surfaces are rough at some level of magnification.

In (1) we are lowering a block, magnified until a bump shows.

In (2) the block touches the table and the bump is the forst point to touch (there must be one)

All the weight of the block is now resting on that one point, so it deforms as it cannot support such a load, as in (3)

This continues untill a second (third, fourth etc) bump joins and the total surface area of the bumps is just sufficient to support the block as in (4)

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Do you mean

1. there are many bumps

2. One point touches and deform

3. Second point touches and deform and so on

4. until TSA of bumps defom to a point where the deformation can support the blocks weight?

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That's exactly right!

Edit : Did you want to discuss it further?

Edited by studiot
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I doubt this can be simplified that much. Imagine the dynamics involved in aviation. The range in speeds from takeoff to supersonic and the pressures that are involved everywhere in between these two boundaries. Materials change and respond greatly due to the degree of energy that is being transferred. A jump off a dock as compared to one from a high bridge for example, the same water, two very different results. Materials can stiffen with increasing pressure while others will deform and still others will disintegrate. The considerations of varying temperatures and pressures for each material and their VERY particular and unique dynamic interaction with all other materials . . . . well, just staggers the mind.

the point i was making for the young lady is to try experimenting herself.

the idea behind making an experiment is isolating things out and removing them until you get an answer.

to tell her she shouldnt try is to tell her that she should not consider experimenting and instead concoct an idea with no real foundation.

we dont need another one of them...

Do you mean

1. there are many bumps

2. One point touches and deform

3. Second point touches and deform and so on

4. until TSA of bumps defom to a point where the deformation can support the blocks weight?

i would also lke to mention that mass does not interact well.

when two nucleuses touch sparks fly and you have by products.

this means that charges are involved and mediate any and all interactions below a certain point.

beyond that point the strong force takes over and BANG!

at the classic scale things seem solid because of this very force responsible for the charges.

remove the charges and the materials will pass right through each other.

this is how i know it for sure.

however you are better off using basic physics to describe objects at the classic scale.

in other words all structure and interactions between them is mediated directly by charges.

1

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I will stay with you and continue to discuss this topic. It is good to be skeptical, studiot. I like the words 'citations and references needed'.

when two nucleuses touch sparks fly and you have by products.

this means that charges are involved and mediate any and all interactions below a certain point.

beyond that point the strong force takes over and BANG!

remove the charges and the materials will pass right through each other.

in other words all structure and interactions between them is mediated directly by charges.

Can you describe it in detail but not just animation in words like BANG! and done? I mean more strict, formal and quite professional with scientific jargons way of saying. I am interested in modern physics. I would like to know how elementary particles will affect the frictional force. Anyway, i don`t get the idea from the 'animation in words' posted by you in the previous post. Please explain in detail, thanks.

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• 5 weeks later...

could you explain why there is heat from friction

could it be a transefer of kentic energy from the objects moving to thermal energy. Energy can not be destryed but only transfered so could this be a relase of the energy being stored up from the resistance of the to objets rubbing each other??

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could you explain why there is heat from friction

could it be a transefer of kentic energy from the objects moving to thermal energy. Energy can not be destryed but only transfered so could this be a relase of the energy being stored up from the resistance of the to objets rubbing each other??

Good question, but I'm afraid an answer is rather more complicated.

Any theory of heat generation from friction must explain the following facts.

1. The force of static friction does not generate heat, only dynamic friction results in heat generation.
2. An object of mass, m, that is moving at velocity, v, has the same kinetic energy regardless of whether it is rubbing against something or not.
3. If I fix something down to the workbench so that it does not move and rub something past it, both the moving and fixed objects experience a rise in temperature.
4. The standard coefficient of dynamic friction, and therefore the standard dynamic frictional force is usually independent of the relative velocity.

Obviously the heat energy ultimately comes from whatever supplies the source of motion, but the mechanism is another thing entirely.

Edited by studiot
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The standard coefficient of dynamic friction, and therefore the standard dynamic frictional force is usually independent of the relative velocity.

In the usual bad books, yes.

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